Use a Mac now and Thunderbird is using 0.1% CPU idle and 562MB.
I use the "folders on the left, message list on the right top, selected message view underneath" view.
Things that I require on Thunderbird (that I had to hack, muck around with):
* Timestamps in ISO8601
* Either the ability to use a Solarized theme, or one that accurately uses the MacOS light/dark themes.
* Larger (16-20pt) font sizes for all panes
* Really good integration between a CardDAV server (I use fastmail) and accumulating email addresses automagically and not get confused with my local MacOS Contacts.
I don't use the tasks/events/calendar integration because it gets confused between my CalDAV server (fastmail), and my local Mac OS calendar.